This is pretty freaky. Recently when I got the redmi 3s prime for dad, I had to get his SIM changed. Vodafone Store guy gave me a new sim without any paperwork. Told me to send the sms in this format to switch the connection from the old sim to the new sim. The process took less than 2 minutes. No verification from Vodafone. NOTHING.
This probably explains why operators are being forced to send this warning to their subscribers. They have made the process of sim swap so easy that it is being abused by scammers. Just one SMS would handover your mobile number to another person. And only god knows what he can do with it. Changing passwords of digital wallet accounts would be rather simple. Bank accounts would be slightly tough. I bet there are tens of other things that I cannot recollect right now.
This also means that one has to be extremely careful with their phones. Anyone can take over your mobile number if they can get hold of your phone for even a minute. To gain back your number, I assume you would need to run to a company outlet. Not sure if you can get your SIM blocked through a phone call to the customer support center.